Ink composition comprising optically variable pigments, use of the composition, optically variable pigment and method of treating said pigment

ABSTRACT

The invention relates to passivated optically variable pigment, a method of preparing said passivated optically variable pigment, an ink composition comprising said passivated optically variable pigment, the use of said ink composition, and a document carrying a marking made with said ink composition.

[0001] The invention relates to passivated optically variable pigment, amethod of preparing said passivated optically variable pigment, an inkcomposition comprising said passivated optically variable pigment, theuse of said ink composition, and a document carrying a marking made withsaid ink composition.

[0002] Inks containing optically variable pigment as an overt securityfeature are widely used on bank notes and value documents, in order toprotect them from counterfeiting by generally available colorreproduction equipment such as color copiers, scanners and printers.

[0003] A common type of optically variable pigment is based on a layeredoptical interference structure. The interference structure typically hasat least one metallic reflecting layer, at least one transparentdielectric layer and at least one semitransparent metal layer. Metalslike aluminum, gold, copper or silver are used as the metallicreflecting layer, chemical compounds like magnesium fluoride, silicondioxide or aluminum oxide are used as the transparent dielectric layerand metals like chromium or nickel are used as the semitransparent metallayer.

[0004] Incident white light is partially reflected at the pigment'ssemitransparent surface layer, and partially at the underlying metallayer. The difference in optical path between both parts of reflectedlight results in constructive or destructive interference, depending onthe wavelength, i.e. enhances the reflectivity for certain wavelengthsand reduces it for others. This spectral discrimination is perceived bythe human eye as the appearance of color. For different angles of view,the difference in optical path changes, which makes the layered materialexhibit angle-dependent color.

[0005] Optically variable pigments are usually manufactured by vaccumdeposition of the different required layers onto a flexible web. Afterdeposition of the desired number of layers, the stack of layers isremoved from the web, either by dissolving the web in a suitablesolvent, or by stripping the optically variable material from the web.The optically variable material is then broken down to flakes which haveto be further processed to fit the proposed application, by e.g.grinding, milling, etc. The resulting product consists of flat flakeswith broken edges and irregular shapes and different aspect ratios. Theflakes have two planar, parallel surfaces showing said interferenceproperties.

[0006] The term “aspect ratio” defines the ratio between the extensionof the flake in the planar dimensions and the thickness of theinterference layer stack. The former is generally of the order of 5 to40 μm, whereas the latter is generally of the order of 1 μm.

[0007] A practical embodiment of the optically variable pigment flake isbased on a symmetric Cr/MgF₂/Al/MgF₂/Cr stack, wherein the thickness ofthe chromium absorber layers is 3.5 nm, that of the MgF₂ dielectriclayers is between 200 and 600 nm, and that of the aluminum reflectorlayer is about 60 nm. The chromium surface layers constitute furthermorean efficient protection of the underlying MgF₂ and Al layers againstchemical attack.

[0008] In the area of broken edges, however, the inner layers of thestack are accessible and not covered by any protective layer. Forenvironmental reasons water-based ink formulations are now widely usedand required. However at the pH value of water-based inks, corrosion ofcertain materials of the optically variable pigment can occur.

[0009] For example, ink formulations containing water-borne acrylicemulsions have generally a pH value in the range of 7.0 to 8.5. Underthese conditions aluminum may be attacked, in particular in the presenceof carboxylic groups and other chemical agents which form complexes withthe Al³⁺ ion. Simultaneously hydrogen gas is liberated, blowing up theflakes' interference structure, destroying the optically variable coloreffect. The magnesium fluoride of the dielectric layers can as well bedissolved by water, which also destroys the interference pigment, andthus the optically variable color effect.

[0010] U.S. Pat. Nos. 5,527,848 and 5,658,976 describe passivation ofoptically variable pigments by treating the pigment flakes withtransition metal and rare earth metal salt solutions, which create athin coating on the pigment surface. U.S. Pat. Nos. 5,545,677, 5,552,458and 5498781 and EP 0688833 describe the passivation of opticallyvariable pigments by modifying the pigments in a chemical reaction witha silane functional group. These modified pigments are used forpreparing a pigmented coating formulation.

[0011] It is an object of the present invention to protect opticallyvariable pigment of the mentioned type, in order to reduce or inhibitthe oxidation of its metal layers and the dissolution of its dielectriclayers. The chemical nature of the different materials of themulti-layered Cr/MgF₂/Al/MgF₂/Cr construction of said optically variablepigment requires noteworthy a specific selection of the passivatingagent.

[0012] It is a further object of the present invention to use suchprotected pigments in ink compositions. A particular object of thepresent invention is a water-based screen printing ink formulationcontaining an passivating system for said optically variable pigments.

[0013] These and other objects are achieved by the invention inaccordance with the independent claims.

[0014] An ink composition in accordance with the invention comprises anorganic binder system, water, and a pigment selected from the group ofinterference pigments comprising a layered stack of different materialswherein at least one of the layers is a reflective layer having at leastone chemically exposed surface and at least one of the layers is adielectric layer having at least one chemically exposed surface, andsaid materials comprise one or more metal and/or inorganic metalcompounds, said metal and/or inorganic metal compound being corrosionsensible and wherein at least the chemically exposed surface of saidreflective and said dielectric layer at the edge of said stack of layersis substantially covered by a passivating agent, which is selected fromthe group of anionic tensides.

[0015] The term “tenside” describes chemical compounds which combine twodifferent types of chemical functionalities, i.e a hydrophobic part,called the “tail” of the tenside, and which is soluble in in solventswith low polarity (such as hydrocarbons), and a polar or hydrophilicpart, called the “head” of the tenside, which is soluble in solventswith high polarity (such as water). The “head” of the tenside can becharged (anionic or cationic) or it can be without charge. A tenside mayalso have more than one head, and/or more than one tail.

[0016] The tensides are thus able to solubilize polar entities in anon-polar medium by assembling on the surface of the polar entity, withthe polar head of the tenside pointing to the entity and the non-polartail of the tenside pointing to the non-polar medium. In a similar way,tensides can also solubilize non-polar entities (such as grease) in apolar medium (such as water).

[0017] A tenside may consist of a phosphoric acid group as the “head”and an organic chain (e.g a hydrocarbon or a fluorinated hydrocarbon) asthe “tail” or the “tails”. Said tail groups can be attached to thephosphoric acid by esterification, yielding phosphates. Phosphoric acidcan provide up to three hydroxyl groups for esterification. Furthermore,partly esterified phosphoric acid acts as a buffer via the amount ofprotonated and deprotonated hydroxyl groups. This allows to use them aspH-control-agents.

[0018] The hydroxyl groups and the oxygen atom of phosphates arefurthermore able to act as complex-forming agents toward electrophilicmetal ions. “Complex-forming” means hereby an electrostatic interactionbetween a nucleophilic ligand (hydroxyl groups and/or oxygen atom of thephosphoric acid) and an electrophilic cation, such as H⁺, Mg²⁺, Al³⁺,etc., resulting in a chemical bonding (attachment) of the ligand to thecation. Attachment of a ligand to a cation can result either in amolecular complex, where a cation is completely surrounded by ligands,or in a surface complex, where a cation is part of a solid surface(oxide, fluoride, etc.), having its free, exposed side occupied by oneor more ligands.

[0019] Preferred passivating agents for optically variable pigments ofthe types mentioned before are found in the group of organic esters andfluorinated organic esters of phosphoric acid. These phosphoric acidesters (phosphates), which are known and commercially available assurfactants, were surprisingly found to exhibit excellent attachmentproperties to the different materials of optically variable pigments ofthe Cr/MgF₂/Al/MgF₂/Cr type.

[0020] In a preferred embodiment of the invention, the ink compositioncomprises optically variable pigments wherein the reflecting layers areselected from the group of metals comprising Al, Fe, Ni, Cr, Zn. Thesemetals exhibit suitable properties for the preparation of the opticallyvariable pigments and further excellent properties as reflecting layers.The mentioned phosphate tenside molecules are noteworthy able to firmlyattach to these metal ions via the phosphate “head”, and thus to protect(passivate) the optically variable pigment flake against further attackby the reactive chemicals from the surroundings (ink formulation) bytheir hydrocarbon or fluorohydrocarbon “tails”.

[0021] The passivation of the optically variable pigment can be done intwo ways. In a first way, the ink composition comprises the passivatingagent, and the untreated optically variable pigment is directly added tothis ink composition. However it is also possible, in a second way, topre-treat the optically variable pigment with the passivating agent,prior to incorporating the pigment into the ink composition.

[0022] In both cases, according to the invention, it was found thatadding a surplus of passivating agent to the ink composition isbeneficial, in order to protect any fresh surface which, for example,may appear during a mixing operation.

[0023] Preferably, according to the invention, the ink compositioncomprises optically variable pigments having dielectric layers selectedfrom the group of inorganic metal compounds consisting of MgF₂, Fe₂O₃,Cr₂O₃, MgO, SiO₂. The metal cations of these compounds were found toattach firmly to the phosphate head groups of the tenside.

[0024] The invention allows therefore the use in corrosive medium suchas water-based ink compositions, of highly reflective but corrosionsensible pigments.

[0025] Particularly stable passivation can be achieved if the inkcomposition comprises a passivating agent selected from the group oforganic esters and fluorinated organic esters of phosphoric acid whichhave the following generic structural formula:

(Rf—CH₂—CH₂—O)_(x)P(O) (OH)_(y)

[0026] wherein:

[0027] Rf=F—(CF₂—CF₂)_(z)

[0028] x=1 or 2

[0029] y=2 or 1

[0030] x+y=3

[0031] Z=1 to 7

[0032] Subscript x indicates the number of tails of the tensidemolecule; subscript y indicates the number of hydroxyl groups availablefor complex formation with metal ions. The sum of the subscripts x and yis always three. The selection of x and y also defines the properties ofthe head group of the tenside. The subscript z indicated the number of(CF₂CF₂) entities which are connected to the CH₂CH₂O unit linking thetail to the head group of the tenside. The choice of z further selectsspecific properties of the tenside regarding solubility in differentsolvents. Tenside molecules of the described structural formula, havinga z ranging from one to seven, were found to have suitable propertiesfor the proposed use as a passivating agent for optically variablepigments in ink formulations.

[0033] In an preferred embodiment of the invention, an ink compositioncontains the passivating agent in an amount of 0.5 to 15% w referred tothe weight of the optically variable pigment. More preferred is anamount of 1.5 to 6.5% w and an even more preferred amount is 2.5 to 5.0%w of the weight of optically variable pigment. These amounts ofpassivating agent were found to be sufficient to cover the pigments atleast with a double layer of tenside molecules, and are therefore ableto shield the metal or dielectric layers of the pigment efficientlyagainst the corrosive environment of the ink formulation. Similaramounts as decribed were also found to be sufficient in the directtreatment of the optically variable pigment, in order to cover itsactive surface and to obtain neat, passivated optically variable pigmentfor different kinds of applications.

[0034] Another aspect of the invention is an ink composition comprisingpassivated optically variable pigment and having a pH value between 7.0to 9.0. Preferred is a pH value of the ink composition between 7.3 and8.5 and more preferred a pH between 7.5 to 8.0. The selected pH valueallows the use of lower amounts of passivating agent while maintainingthe corrosion stability of the optically variable pigments and stillmaintaining excellent properties of the ink composition prior toprinting.

[0035] A further embodiment of the invention is an ink compositionwherein the passivating agent is dissolved in an organic solvent. Theuse of dissolved passivating agent provides a better availibity of thetenside molecules to cover the surface of the optically variablepigments and therefore an improved covering of the surface.

[0036] In a preferred embodiment of the invention the organic solventfor dissolving the passivating agent in the ink composition is selectedfrom the group of glycol ethers or the group of glycols. These compoundsprovide for an excellent solubility of said tensides.

[0037] A further aspect of the invention is an ink compositioncomprising passivated optically variable pigment and a binder systemwhich comprises an acrylic or urethane acrylic copolymer emulsion, acrosslinker, optionally a catalyst and optionally further additives. Theacrylic or urethane acrylic copolymer emulsions are selected in such away that the emulsion is alkali soluble. This selection allows tomanufacture a stable ink composition without the risk of precipitationof the copolymer emulsion from the composition.

[0038] Furthermore, an acrylic or an urethane acrylic copolymer emulsionis selected which has a T_(g) value in the temperature range of −10° to50° C. The “glass transition” value T_(g) defines the temperature rangewithin the emulsion will change from an almost solid or highly viscousstate (glass-like) to a low viscous state (fluid-like). The inventorshave found that the T_(g) value has an important influence on theprocessability of the ink composition during printing.

[0039] The “crosslinker” is a component which is able to build up athree-dimensional polymer network by reacting either with furthercomponents of the ink composition, or with other crosslinker molecules.“Curing” in the context of this specification means the drying orsolidifying or polymerizing reacting of the printed ink after printingin such a manner that the ink can i) no longer be removed from thesubstrate and ii) does no longer tack to other substrates placed uponthe printed ink. Additionally the curing effects a passivation of theprinted ink against various kinds of treatments (water, solvents, acids,bases, etc.) within specified limits.

[0040] The term “grafting” means the stable bonding of the crosslinkermolecule to the polymer molecules of the acrylic or urethane acryliccopolymer emulsions. The modified (grafted) molecules will have almostthe same physical properties as before the grafting reaction.

[0041] The “catalyst” is a chemical compound which lowers the activationthreshold for a specified type of reaction and therefore promotes thesaid reaction. The catalyst will remain in the same chemical compositionafter the reaction as prior to it. Due to this fact a catalyst isrequired only in small amounts.

[0042] “Additives” comprises those compounds and materials which areused for adjusting physical and chemical parameters of the inkcomposition, such as the pH value, the viscosity, the consistency, thefoaming properties, the lubricating properties etc.

[0043] In a preferred embodiment, the ink composition comprises aacrylic or urethane acrylic copolymer emulsion of the binder systemwhich is selected from the group of polymers having self-crosslinkingproperties. These properties open the possibility to build aninterconnected network which encloses the pigment particles in such away that they get enhanced resistance against chemical and physicaltreatments.

[0044] In a further preferred embodiment, the ink composition comprisesa crosslinker of the binder system which is selected from the group ofsubstituted alkoxy silanes (R₁)_(y)(R₂O)_(z)Si (wherein R₁, R₂ aredifferent substituents, y+z=4), preferably from the group ofmonosubstituted trialkoxy silanes (y=1, z=3). The substituents R₁, R₂ ofthe crosslinker comprise two different chemical functionalities whereinthe first functionality R₁ is selected in such way that it reacts priorto printing and wherein the second functionality R₂ is selected toeffect curing of the printed ink.

[0045] The first functionality provides the possibility for grafting theacrylic or urethane acrylic copolymer emulsion with a crosslinkingmolecule which is able to react in a second step, upon a furtherinitialization (second functionality). This can be done by a short raiseof temperature which initiates the release of protons by thedecomposition of compounds introduced for the neutralization of theemulsion, and which starts in consequence the curing of the printed inkfilm. “Chemical functionalities” means that a chemical compound containsa group of atoms which undergo a preferred specific type of reaction,e.g —OH or —SH groups are able to react with acids to form esters withthe help of a catalyst. The various chemical functionalities are wellknown to a person skilled in the art. Using a selected choice ofconditions (e.g. temperature, solvent etc.) the skilled in the art isable to control the reaction of chemical compounds which contain morethan one chemical functionality in such a way that only one of thechemical functionalities reacts.

[0046] In an even more preferred embodiment of the ink composition, thecrosslinker of the binder system is selected from the group ofmonosubstituted triethoxysilanes, preferably from the group ofepoxy-cycloaliphatic triethoxysilanes and from the group ofglycidyl-triethoxysilanes. An ethoxy group as substituent R₂ provides areactive group which can be hydrolysed under controlled conditions andwhich is susceptible to react with other components of the inkformulation or with the substrate The epoxy entity as substituent R₁ isable to react with functional groups of the acrylic or urethane acryliccopolymer emulsion, creating a preformed network prior to the printingprocess.

[0047] An additional aspect of the invention concerns an ink compositionwhich comprises an amount of crosslinker in a range between 0.25 and3.0% w referred to the total weight of the composition. A preferred inkcomposition comprises an amount of crosslinker between 0.5 and 2% w andan even more preferred ink composition comprises an amount ofcrosslinker between 1 and 2% w. The described amounts were found toimpart sufficient resistance to the printed and cured ink.

[0048] Preferably the ink composition contains optically variablepigment in amounts ranging between 10 to 25% w of the total weight ofthe composition. An ink composition with an amount of optically variablepigment of 12 to 20% w is preferred and an amount of 15 to 18% w is evenmore preferred. The disclosed amounts of optically variable pigmentyield an ink composition which exhibits an excellent color coverage andoffers the possibility of easy visual and/or machine detection of theoptical properties of the printed and cured ink.

[0049] According to the invention optically variable thin-filminterference pigments are used which are characterized in that thesurface of said pigment is covered with a passivating agent. Saidpassivating agent is selected from the group of anionic tensides, and ina preferred embodiment, selected from the group of organic esters andfluorinated organic esters of phosphoric acid (phosphates). As alreadymentioned above, the tenside primarily acts as a mediator betweenhydrophilic and hydrophobic components, and can thus, for example,solubilize grease in water or vice versa. In addition of beingsurfactants, the phosphate head group of the tenside is a goodcomplex-former and therefore susceptible for attaching itself to metalions and to metal-ion-containing surfaces.

[0050] A further aspect of the invention concerns a method ofpassivating optically variable pigments, said method including thefollowing steps:

[0051] a) provide a passivating agent or a solution of that saidpassivating agent and dissolve it in an organic solvent;

[0052] b) add water to the resulting solution of step a) and mixthoroughly;

[0053] c) adjust the pH of the mixture to a value between 7.3 and 8.5;preferably between 7.5 and 8.0;

[0054] d) disperse optically variable pigment to the mixture obtained instep (c).

[0055] The dissolution of the passivating agent in an organic solventfollowed by the addition of water and pH-adjustment provides asolution-dispersed tenside. The adjustment of the pH-value prior to theaddition of the optically variable pigment avoids or reduces the risk ofa possible reaction of the acidic form of the tenside with the opticallyvariable pigment. This method allows to reduce to a minimum the amountof necessary tenside in order to passivate the surfaces of the opticallyvariable pigment.

[0056] In a preferred embodiment of the invention, the method ofpassivating optically variable pigments comprises the use of saidpassivating agent in amounts ranging between 0.5 to 15% w referred tothe total weight of the optically variable pigment. More preferredamounts are between 1.5 to 6.5% w and even more preferred amounts arebetween 2.5 to 5.0% w. As already mentioned above, these amounts allowfor an excellent coverage of the surface of the pigment flakes.

[0057] Another aspect of the invention concerns a method of passivatingoptically variable pigments, whereby the organic solvent is selectedfrom the group of glycol ethers or from the group of glycols. Thesetypes of solvents were found to allow for suitable solvation of thetenside.

[0058] Still another aspect of the invention concerns a method ofpassivating optically variable pigments whereby the pH value of thesolution containing the passivating agent is preferably adjusted to pH7.3 to 8.5 and more preferably adjusted to pH 7.5 to 8.0. These valueswere found to result in minimal amounts of corrosive species, i.e.phosphoric acid and hydroxide ion, populating the passivating solution,and allow therefore to use a minimal amount of passivating agent withrespect to the optically variable pigment and still to achieve anefficient passivation.

[0059] An additional aspect of the invention concerns the use of saidink composition comprising said passivated optically variable pigmentsfor water-based screen, flexo or gravure printing. Traditional thoseprinting techniques are noteworthy tied to large amounts of organicsolvents which are added to the ink in order to obtain the required lowviscosity for application, and which must be evaporated after printing.Water-based inks rely on polymer emulsions in water, and avoid thereforepotential health-hazards to the printing workers, whilst being at thesame time environment-friendly. Water-based ink formulations are almostincompatible with optically variable pigments, in that they have a veryshort shelf life due to pigment degradation. The present inventionallows to formulate water-based inks containing optically variablepigments whilst having comparable shelf life to solvent based inkscontaining the same pigments.

[0060] A further aspect of the invention concerns a marking on adocument which is obtained by screen, flexo or gravure printing the inkcomposition of the invention. Markings containing the passivatedoptically variable pigment exhibit excellent resistance against chemicaland physical agents, compared to similar markings containing the same,but non-passivated optically variable pigment. The markings containingpassivated optically variable pigment exhibit as well improved opticalfeatures (such as indicated by the measured values of chroma and colorshift) compared to markings containing optically variable pigments whichare not passivated. The long-term color stability of the printingscontaining passivated optically variable pigment is improved as well.

[0061] An additional aspect of the invention concerns a documentcarrying a marking obtained by printing an ink according the invention.

[0062] The invention will now be explained further by non-limitingexamples regarding to the passivation of optically variable pigments andto ink compositions which are given for purposes of illustration.

[0063] List of abbreviations:

[0064] Imicure EMI-24 (Air Products) 2-Ethyl-4-methylimidazole

[0065] AMP-95 (Angus Chemie GmbH) 2-Amino-2-methyl-1-propanol 95%solution

[0066] DMA Fluka (N,N′-dimethylethanolamine)

[0067] Neocryl XK-11 NeoResins/Avecia

[0068] Neocryl XK-14 NeoResins/Avecia

[0069] Neocryl BT-9 NeoResins/Avecia

[0070] Neocryl BT-20 NeoResins/Avecia

[0071] Armorez CR2900 Westvaco

[0072] CoatOSil®1770 Witco Co. beta-(3,4-Epoxycyclohexyl)ethyltriethoxysilane

[0073] CoatOSil® Y-11988 Witco Co 40% emulsion of CoatOSil® 1770 inwater

[0074] CX-100 NeoResins/Avecia polyaziridine compound

[0075] Zonyl® UR Dupont Fluorosurfactant

[0076] Passivation of Optically Variable Pigments

[0077] The optically variable pigments (OVP) used in the describedexamples comprise 3 different materials, noteworthy a thin film ofaluminum (Al), a dielectric layer of magnesium fluoride (MgF₂) and avery thin layer of chromium (Cr). Aluminium is known to be attacked bywater under alcaline conditions, according to the chemical equation:

2Al+6H₂O+2OH⁻→2[Al(OH)₄]⁻+3H₂.

[0078] Magnesium fluoride, MgF₂, in turn, is slightly soluble in water.Qualitative and quantitative analyses of the water-soluble residues (Mg,Al, Cr) of the optically variable pigments were obtained using atomicabsorption spectrometry, an analytical method well known to the skilledin the art. Concentrations of soluble Mg, Al and Cr were thus measuredfrom supernatant solutions obtained after dispersing optically variablepigment in water at pH=8.5. The evolution of said concentrations wassubsequently followed over a time period of 2 months.

EXAMPLE I

[0079] Direct Passivation in Water:

[0080] 1.4 g of the acidic form of passivating agent Zonyl®UR aredissolved in 10 mL of butylglycol. The solution is diluted up to 100 mLwith deionised water. The pH is adjusted with DMA(N,N′-dimethylethanolamine). 5 g of OVP are dispersed in 95 g of theabove described solution (Sample 1) at T=25° C. The OVP dispersion isfiltered after 1 day, 1, 2, 3 weeks and 2 months. Filtrates are dilutedup to 200 mL, analysed by AAS. As a reference the same procedure isperformed on non-passivated OVP (Reference 1).

[0081] Results of the AAS analysis of passivated and non-passivated OVPin solution of pH 8.5 at varying reaction times. t/days 1 7 14 21 60passivated OVP (Sample 1) Mg* 3.0 3.5 4.0 4.0 6.0 Al* 0.84 0.28 0.620.60 2.11 Cr* 1.22 1.49 1.43 1.38 1.51 non-passivated OVP (Reference 1)Mg* 24.0 27.0 25.0 27.0 31.5 Al* 1.67 0.91 0.98 0.60 1.03 Cr* 0.02 0.010.03 0.05 0.00

[0082] The passivating agent reduces the Mg and Al concentration at a pHvalue of 8.5 compared to the non-passivated samples, except for the Alconcentration at 60 days reaction time. The Cr concentration is higherthan the OVP which has not been passivated. The passivating agent seemsto complexate and stabilize the Cr ions in the solution.

[0083] The molecules of the passivating agent arrange themselves inmulti-layers and thus create an hydrophobic barrier which avoidmolecules of water to reach the OVP surface and hydrate MgF₂. It isbelieved that water molecules when hydrating MgF₂ would cause swellingof the dielectric layer and impair or destroy the optically shift. Thesemulti-layers further avoid Al oxidation caused by attack of hydroxideions.

EXAMPLE II

[0084] 0.5 g of Zonyl®UR is dissolved in 6 g dipropyleneglycolmethylether and completed to 100 g with deionised water. AMP-95 is addedto adjust the pH to 8.5 at T=25° C. 15 g of green-to-blue OVP aredispersed with 85 g of that solution (Sample 2). A reference is preparedwithout passivating additive at pH=8.5 (Reference 2). After 24 hours,the dispersion is filtered, washed thoroughly with deionised water, butnot dried, and the OVP sample is re-dispersed in solutions of the samepH:1. with Zonyl®UR (sample 3) and 2. without Zonyl® UR (sample 4).Concentrations of Mg, Al and Cr are measured using AAS after 24 hoursand 2 weeks according to the same procedure.

[0085] Results of the AAS analysis of samples 2 (S2), 3 (S3), 4 (S4) andreference 2 (R2) at varying reaction times. Mg*/pH = 8.5 Al*/pH = 10Cr*/pH = 8.5 S2 S3 S4 R2 S2 S3 S4 R2 S2 S3 S4 R2  1 d 3.0 10.5 11.0 10.04.72 4.19 9.58 8.82 1.22 0.02 0.01 0.02 14 d 4.0 19.25 19.0 15.0 4.222.09 1.56 1.0 1.43 0.04 0.02 0.03

[0086] The described method uses a ratio of Zonyl®UR to OVP green/bluewhich is six times lower than compared to sample 1 therefore theconcentration values are different.

[0087] The fine dispersion of metallic Cr in the form of nanoscopicflakes or clusters acts in the presence of passivating agent as a veryeffective wetting and dispersing aid. These nanoparticles of Cr comefrom the “dust” caused by the crunching operation during OVPmanufacturing. The passivating agent detach the “dust” from the OVPsurface and disperse it in water in a very effective way. Thenanoparticles are too small to be filtered off. Cr is no longer presentin solution when OVP is filtered, washed and placed again in the sameconditions. Nevertheless the Cr concentration is very low [Cr]<1.5 ppm.

EXAMPLE III

[0088] Indirect Passivation in a Non-Aqueous Solution

[0089] 0.5 g of Zonyl®UR are dissolved in 6.0 g of dipropyleneglycolmethylether and 0.14 g of AMP-95 are added to neutralise the additive ofpassivation. This solution is added to the OVP (15 g). Mixing is donemanually in order to achieve a good wetting and the mixture is left for24 hours. 7.22 g of this mixture which contains 5 g of pure OVP is addedto a water solution (total: 100 g) at pH=8.5 and T=25° C. The dispersionis filtered off after 24 hours and 2 weeks, solutions are completed to200 mL and analysed by AAS (Sample 5).

[0090] Results of the AAS analysis of samples 1 (S1) and 5 (S5) andreference 1 (R1) at varying reaction times. Mg*/pH = 8.5 Al*/pH = 10.0Cr*/pH = 8.5 S1 R1 S5 S1 R1 S5 S1 R1 S5  1 d 3.0 30.2 14.5 4.72 0.881.07 1.22 0.0 0.03 14 d 4.0 19.75 18.25 4.22 0.42 1.22 1.43 0.0 0.03

[0091] With an equal concentration of OVP, at pH=8.5, the initialconcentration of Mg (14.5 ppm) is largely higher after 24 hours comparedto sample 1 (3 ppm), but significantly lower to a reference withoutpassivation (30.2 ppm). Concentrations of Mg tend to reach the samevalue after 2 weeks (19.75 ppm).

EXAMPLE IV

[0092] Influence of Different Concentrations of Zonyl®UR:

[0093] 0, 0.1, 0.25, 0.5, 1 and 2 g of Zonyl®UR are added to 6 g ofdipropyleneglycol methylether, the pH value is adjusted to 8.5 withAMP-95. Water is further added to complete the solution to 85 g. 15 g ofOVP green/blue (sample 6) or 15 g of Chromaflair® magenta/gold (sample7) are dispersed in this solution. Passivated OVP or passivatedChromaflair® is filtered off after 24 hours and 2 months respectively,the solutions are completed to 200 mL and analysed by AAS. TheChromaflair® pigments have been thermally treated to stand hydrolysis indifficult weather conditions (developed for the car industry).

[0094] Resulting Mg Concentration for Different Amounts of Zonyl®UR atpH 8.5 and at T=25° C.: 0% 0.1% 0.25% 0.5% 1% 2% Sample 6  1 day 24.06.0 3.5 2.0 2.5 4.0 60 days 31.5 12.32 10.33 5.82 3.28 3.78 Sample 7  1day 9.5 3.1 1.1 1.6 2.2 4.1 60 days 34.88 23.88 9.71 6.5 2.13 3.78

[0095] The concentration of Mg is dependent to the Zonyl®URconcentration as it can be seen from the table. The optimumconcentration is comprised between 0.25 and 1%, ideally at 1.5%. TheOVP/Zonyl®UR ratio should be kept between 1.5 and 6.5.

[0096] For OVP with 0.5% of Zonyl®UR, the concentration of Mg is dividedby 12 within 24 hours. After 2 months the concentration of Mg increasesbut it is still five times lower with 0.5% of Zonyl®UR and eight timeswith 1% than without Zonyl®UR.

[0097] For Chromaflair®, with 0.5% of Zonyl®UR the concentration of Mgis divided by 6 within 24 hours. After 2 months the concentration of Mgincreases but it is still 5 times lower with 0.5% of Zonyl UR and 16times with 1% than without Zonyl®UR.

[0098] The concentration of Mg is two times lower with a Chromaflair®pigment compared to an OVP pigment.

EXAMPLE V

[0099] One component water-based ink compositions with passivatedoptically variable pigments for rotary screen press application.

[0100] Water-based screen inks with color-shifting or optically variablepigment are applied on security banknote paper using an automatichand-coater (hand-coater bar no 3, wet film thickness of 24 μm). Theapplied ink is dried for 30 seconds at 80° C. and the adhesion ischecked with a finger nail. Chemical and physical resistances arenormally measured with solvents, acid and alkali bleeding at RT orelevated temperature, wet crumpling (=WC) and dry crumpling (=DC), wetrub (=WR) and dry rub (=DR), laundry tests (specifications asestablished by INTERPOL at the 5^(th) International Conference onCurrency and Counterfeiting in 1969, or to the Bureau of Engraving andPrinting's test methods as stated in BEP-88-214 (TN) section M5).Resistances to laundry, wet rub and alkali bleeding are normally themost difficult to achieve. The wet and dry crumpling tests wereperformed on a IGT instrument. A print by approx. 5×5 cm is rolled andintroduced in a tube. The roll of paper is crunched in the tube using apiece of metal which has the same diameter. The piece of paper isflattened and rolled again in an other direction (by 90°). After 4×, theprint is turned on the other side. The operation is repeated 4×(wet) or8×(dry). The wet test is done in the same conditions using a print whichhas been damped in water for 10 minutes.

[0101] Rub tests were pursued with a Prüfbau instrument. Conditions dryrub tests, 100× with a 610 g weight and wet rub tests after havingdamped samples for 10 minutes in water.

[0102] The first laundry test, called laundry solution with stirring, isperformed using a 1 L reactor, comprising a mechanical stirrer, aheating mantle, containing 500 mL of water, 2.5 g of industrial laundrypowder (Persil, Henkel or equivalent) and 5 g Na₂CO₃. Three printedsamples (squares with 5×5 cm on edge) are placed in the reactor, stirredand heated for 30 minutes. Samples are washed off using distillatedwater and dried for 2 hours at 40° C. The result is estimated on aaverage of three samples. The slow mechanical stirring and hightemperature of this test is the illustration of a specific chemicaltest. The second laundry test, called washing machine test, is performedusing a standard washing machine (Lavamat W 1020, AEG) with 2 kilogramsof cotton fabrics and 100 mL of laundry powder Persil, Henkel). Printedsamples (squares with 5×5 cm on edge) are placed in individual cottonpockets. The laundry test is pursued at 95° C. for 40 minutes. Theresult is estimated on a average of three samples. The good mixing inindividual pockets and the high temperature is the illustration of aspecific physical test.

[0103] The ink alteration is visually estimated according to thefollowing scale: Note Visual ink alteration 6 No alteration 5  0-20% 420-33% 3 33-50% 2 50-66% 1 66-80% 0  80-100%

[0104] Sample 8 and 10 consist of passivated optically variablepigments. Sample 9 and 11 contain the same optically variable pigmentsbut in a non passivated state. The ink samples are prepared followingthe procedure:

[0105] Sample 8:

[0106] 1. Pigment Passivation In Situ in Water:Propylenglycolmethylether 6.0 Zonyl UR 0.5 Water 22.0 OVP green/blue15.0 AMP-95 0.25

[0107] To a solution of Zonyl®UR in propylenglycolmethylether at 50° C.water is added. Further, OVP is added at room temperature when mixingand pH is maintained between 7.5-8.0 with AMP-95. The OVP pigment isdispersed slowly (500 RPM) using a lab mixer for 30 minutes.

[0108] Preparation of the Ink: Neocryl XK-11 48.0 Jonwax 22 3.0 Byk 024(BYK-Chemie) 1.9 Byk 025 (BYK-Chemie) 0.1 Aerosil 200 (BYK-Chemie) 1.0CoatOSil 1770 Witco Co. 1.0 Silwet L-7608 Witco Co. 0.1

[0109] All components are introduced directly into the OVP dispersionand stirred for 5 minutes at 1000-1500 RPM. pH is measured and adjustedto 7.5-8.0 if necessary with AMP-95. CoatOSil®1770 and Silwet L-7608 areadded as a blend to the ink under good mixing at 1500 RPM for 15minutes.

[0110] 2. Viscosity Adjustment: Water 1.7 Rheolat 278 0.55

[0111] The thickener (Rheolat 278) is added carefully in order to obtaina viscosity between 250±50 mPa.s. If necessary, AMP-95 is added tomaintain the pH between 7.5-8.0.

[0112] The same ink is made without the passivation agent Zonyl®UR(Sample 9). An ink using a Chromaflair™ pigment (magenta to gold) withand without Zonyl®UR is also prepared the same way (Examples 10 and 11).

[0113] Results of ink composition with passivated and non-passivatedoptically variable pigments. Sample 8 Sample 9 Sample 10 Sample 11months LWM* LSS* LWM* LSS* LWM* LSS* LWM* LSS* 0 5.5 5.75 4.5 5.25 5.355.25 4.3 5.25 1 2.3 5.7 3.7 5.5 5.2 5.6 4.3 5.3 2 1 5.5 2.3 5.7 4.75 54.1 5.5

EXAMPLE VI

[0114] Stability of an ink composition for rotary screen pressapplication with regard to polymerization.

[0115] The viscosity of samples 8 to 11 are checked after definitetimes. A further series of samples are prepared according to example Vexcept that prior to step three of preparation (viscosity adjustment)the ink composition is aged overnight (samples 12 to 15). 0 d 7 d 14 d30 d 60 d 90 d 25° C. S8 220 735 735 835 870 980 S9 250 915 850 870 930955 S10 260 490 460 480 510 615 S11 250 760 725 785 850 840 40° C. S8220 880 900 1090  1575  2540  S9 250 4150  4525  gel. gel. gel. S10 260600 545 7354  695 850 S11 250 840 820 1660  gel. gel. 25° C. 0 d 1 d 2 d58 d 84 d 198 d S12 290 365 370 435 450 755 S13 265 290 300 355 360 530S14 265 310 320 320 415 700 S15 290 305 340 440 430 690

[0116] Color Shift Stability

[0117] The inks of samples 12 to 15 are kept at 25° C. and applied onbanknote paper using an automatic hand-coater (bar No 3, speed 3). Thefirst color is measured at 0° (specular angle) with illumination at22.5° and the second color is measured at 67.5° with illumination at 45°using a goniospectrometer especially developed for optically variableinks coloristic measurements (Goniospektrometer Codec WI-10 5&5 by PhymaGmbH. Austria). Five measurements on three different prints areaveraged. Coloristic values (L*, a*, b*, C*, h* and ΔE*) are recordedfor each ink example and checked upon aging. The hue h* of OVI™water-based screen inks (the coloristic difference between the inkprinted after n days and the ink printed after manufacturing) are morestable upon aging using a passivating agent than without.

[0118] Results of hue h* measurements. 0 d 1 d 2 d 6 d 13 d 22 d 28 d 56d 84 d First angle (0°) S12 117.23 117.19 116.99 116.82 115.98 116.30116.07 115.89 116.55 S13 116.83 116.13 116.07 114.79 114.81 114.98114.74 114.40 114.80 S14 313.97 313.71 313.33 313.20 312.55 313.43131.06 312.51 313.45 S15 131.19 131.03 312.24 312.31 311.21 312.25312.36 311.06 311.86 Second angle (67.5°) S12 277.01 276.92 276.07276.34 275.37 275.54 275.48 274.90 275.29 S13 276.20 275.71 275.37274.19 273.86 273.87 273.79 273.16 273.15 S14 121.96 121.97 121.18121.03 120.27 120.81 120.43 120.01 121.12 S15 120.36 120.29 119.48119.32 117.88 118.53 118.56 116.97 118.03

EXAMPLE VII

[0119] Preparation of one-component water-based inks with passivated OVPfor flat screen press application.

[0120] Sample 16

[0121] 1. Pigment Passivation In Situ in Water: Proglyde DMM 6.0 ZonylUR 0.5 Water 18.5 OVP green/blue 15.0 AMP-95 0.25

[0122] Zonyl®UR is dissolved in Proglyde DMM(propylenglycoldimethylether) at 50° C. and water is added. OVP is addedat room temperature when mixing and pH is maintained between 7.5-8.0with AMP-95. The OVP pigment is dispersed slowly (500 RPM) using a labmixer for 30 minutes.

[0123] 2. Preparation of the Ink: Tego foamex 800 1.0 Jonwax 22 3.0Neocryl BT-20 50.0 AMP-95 1.0

[0124] Antifoaming additive and wax are added and mix for 5 minutes at1000 RPM. Then, alkali soluble acrylic emulsion Neocryl BT-20 issimultaneously introduced directly into the OVP dispersion with AMP-95and stirred for 5 minutes at 1000-1500 RPM in order to reach a pHcomprised between 7.5-8.0.

[0125] 3. Cross-Linker Addition: CoatOSil ® 1770 2.0 Silwet L-7608 0.2

[0126] CoatOSil®1770 and Silwet L-7608 are added as a blend to the inkunder good mixing at 1500 RPM for 30 minutes. The ink is left overnightbefore viscosity correction.

[0127] 4. Viscosity Adjustment: Aerosil 200 1.0 Water 0.25 Acrysol RM-81.3 Total 100.00

[0128] The thickener (Acrysol RM-8) is added carefully in order toobtain a viscosity 800±50 mPa.s. If necessary, AMP-95 is added tomaintain the pH between 7.5-8.0.

[0129] The same ink is made without the passivation agent Zonyl® UR(Sample 17). Two identical ink formulations are prepared with themagenta-green OVP pigment (Sample 18 with Zonyl®UR and sample 19 withoutZonyl®UR).

[0130] The prepared inks (Samples 16-19) are kept at 25° C. and appliedon banknote paper using an automatic hand-coater (bar No 3, speed 3).The first color is measured at 0° (specular angle) with illumination at22.5° and the second color is measured at 67.5° with illumination at45°. Five measurements on three different prints are averaged.Coloristic values (L*, a*, b*, C*, h* and ΔE*) are recorded for each inkexample and checked upon aging. For example, the ΔE* of OVI™ water-basedscreen inks (the coloristic difference between the ink printed after ndays and the ink printed after manufacturing) are more stable upon agingusing a passivating agent at both view angles

[0131] Results of ΔE* measurements. 1 d 2 d 5 d 7 d 14 d 21 d 28 d 57 d84 d First angle (0°) S16 1.82 1.94 0.96 1.72 1.20 1.54 1.09 2.27 1.67S17 0.98 2.01 2.55 2.67 3.29 3.1 2.69 3.08 2.63 S18 0.53 0.19 1.29 1.481.23 1.99 2.03 1.99 1.36 S19 0.74 0.60 1.78 4.24 5.35 5.45 5.61 5.996.79 Second angle (67.5°) S16 0.89 0.66 2.13 2.22 2.01 3.33 1.74 2.132.14 S17 1.25 1.29 3.81 4.53 6.12 5.35 5.98 6.11 5.93 S18 0.78 0.40 2.461.73 3.71 2.59 3.93 3.58 3.68 S19 1.22 0.85 2.08 3.07 3.64 4.55 4.785.70 5.92

1-25. (Cancelled).
 26. An ink composition comprising a) a binder system;b) water; and c) a pigment selected from the group consisting ofinterference pigments comprising a layered stack of different materialswherein at least one of the layers is a reflective layer having at leastone chemically exposed surface and at least one of the layers is adielectric layer having at least one chemically exposed surface, andsaid materials comprise one or more metal and/or inorganic metalcompounds, said metal and/or inorganic metal compound being corrosionsensible and wherein at least the chemically exposed surface of saidreflective and said dielectric layer at the edge of said stack of layersis substantially covered by a passivating agent, said passivating agentbeing selected from the group consisting of anionic tensides.
 27. An inkcomposition according to claim 26 wherein said passivating agent isselected from the group consisting of organic esters and fluorinatedorganic esters of phosphoric acid.
 28. An ink composition according toclaim 26 wherein the reflecting layers of the optically variablepigments are selected from the group consisting of metals consisting ofAl, Fe, Ni, Cr, Zn.
 29. An ink composition according to claim 26 whereinthe dielectric layers of the optically variable pigments are selectedfrom the group consisting of inorganic metal compounds consisting ofMgF₂, Fe₂O₃, Cr₂O₃, MgO, SiO₂.
 30. An ink composition according to claim26 wherein the passivating agent is selected from the group consistingof organic ester and fluorinated organic ester of phosphoric acid havingthe structural formula: (R_(f)—CH₂—CH₂—O)_(x)P(O)(OH)_(y) wherein:R_(f)=F—(CF₂—CF₂)_(z) x=1 or 2, y=2 or 1, x+y=3, and z=1 to
 7. 31. Anink composition according to claim 26 wherein the passivating agentcomprises 0.5% to 15.0% of the weight of optically variable pigment. 32.An ink composition according to claim 26 wherein the passivating agentcomprises 1.5% to 6.5% of the weight of optically variable pigment. 33.An ink composition according to claim 26 wherein the passivating agentcomprises 2.5% to 5.0% of the weight of optically variable pigment. 34.An ink composition according to claim 26 wherein the pH value of thecomposition is 7.0 to 9.0.
 35. An ink composition according to claim 26wherein the pH value of the composition is 7.3 to 8.5.
 36. An inkcomposition according to claim 26 wherein the pH value of thecomposition is 7.5 to 8.0.
 37. An ink composition according to claim 26wherein the passivating agent is dissolved in an organic solvent.
 38. Anink composition according to claim 26 wherein the organic solvent isselected from the group consisting of glycol ethers and glycols.
 39. Anink composition according to claim 26 wherein the binder system furthercomprises: (a) an acrylic or urethane acrylic copolymer emulsion; (b) acrosslinker; (c) optionally a catalyst; and (d) optionally furtheradditives.
 40. An ink composition according to claim 39 wherein theacrylic or urethane acrylic copolymer emulsion of the binder system isselected from the group consisting of polymers having self-cross-linkingproperties.
 41. An ink composition according to claim 39 wherein thecross-linker of the binder system is selected from the group consistingof substituted alkoxy silanes (R₁)_(y)(R₂O)_(z)Si (wherein R₁, R₂ aredifferent substituents, and y+z=4).
 42. An ink composition according toclaim 39 wherein the cross-linker of the binder system is selected fromthe group consisting of monosubstituted trialkoxy silanes(R₁)_(y)(R₂O)_(y)Si (wherein y=1, and z=3).
 43. An ink compositionaccording to claim 39 wherein the cross-linker of the binder system isselected from the group consisting of monosubstituted triethoxysilanes.44. An ink composition according to claim 39 wherein the cross-linker ofthe binder system is selected from the group consisting ofepoxy-cycloaliphatic triethoxysilanes and glycidyl-triethoxysilanes 45.An ink composition according to claim 39 wherein the weight of thecrosslinker is 0.25% to 3.0% of the total weight of the composition. 46.An ink composition according to claim 39 wherein the weight of thecrosslinker is 0.5% to 2.0% of the total weight of the composition. 47.An ink composition according to claim 39 wherein the weight of thecrosslinker is 1.0% to 2.0% of the total weight of the composition. 48.An ink composition according to claim 26 wherein the weight of opticallyvariable pigments is 10.0% to 25.0% of the total weight of thecomposition.
 49. An ink composition according to claim 26 wherein theweight of optically variable pigments is 12.0% to 20.0% of the totalweight of the composition.
 50. An ink composition according to claim 26wherein the weight of optically variable pigments is 15.0% to 18.0% ofthe total weight of the composition.
 51. An optically variable pigmenthaving a stack of planar layers, said stack of planar layers comprisesat least one reflective layer and at least one dielectric layer, saidreflective layer and said dielectric layer having at least onechemically exposed surface and being corrosion sensible wherein at leastthe chemically exposed surface of said reflective and said dielectriclayer at the edge of the stack of layers of the edge-surface of thepigment is covered with a passivating agent, said agent being selectedfrom the group consisting of anionic tensides.
 52. An optically variablepigment according to claim 51 wherein said passivating agent is selectedfrom the group consisting of organic esters and fluorinated organicesters of phosphoric acid.
 53. An optically variable pigment accordingto claim 51 wherein the passivating agent is selected from the groupconsisting of organic esters and fluorinated organic esters ofphosphoric acid having the structural formula:(R_(f)—CH₂—CH₂—O)_(x)P(O)(OH)_(y) wherein: R_(f)=F—(CF₂—CF₂)_(z), x=1 or2, y=2 or 1, x+y=3, and Z=1 to
 7. 54. An optically variable pigmentaccording to claim 51 wherein the amount of passivating agent is 0.5% to15.0% of the total weight of the optically variable pigment.
 55. Anoptically variable pigment according to claim 51 wherein the amount ofpassivating agent is 1.5% to 6.5% of the total weight of the opticallyvariable pigment.
 56. An optically variable pigment according to claim51 wherein the amount of passivating agent is 2.5% to 5.0% of the totalweight of the optically variable pigment.
 57. A method of treating anoptically variable pigment according to claim 51 comprising the steps:(a) providing a passivating agent dissolved in an organic solvent; (b)mixing water into the solution provided in step a); (c) adjusting the pHof the composition obtained in step b) to a value between 7.0 and 9.0;and (d) dispersing optically variable pigment into the compositionobtained in step c) while keeping the pH value at an appropriate level.58. A method according to claim 57 wherein the amount of the saidpassivating agent comprises 0.5% to 15.0% of the total weight of theoptically variable pigment.
 59. A method according to claim 57 whereinthe amount of the said passivating agent comprises 1.5% to 6.5% of thetotal weight of the optically variable pigment.
 60. A method accordingto claim 57 wherein the amount of the said passivating agent comprises2.5% to 5.0% of the total weight of the optically variable pigment. 61.A method according to claim 57 wherein the organic solvent is selectedfrom the group consisting of glycol ethers and glycols.
 62. A methodaccording to claim 57 wherein the pH value is adjusted to 7.3-8.5.
 63. Amethod according to claim 57 wherein the pH value is adjusted to7.5-8.0.
 64. A method of using an ink composition according to claim 26as a water-based screen, flexo or gravure printing ink, comprising thesteps of providing said ink composition and performing a screen, flexoor gravure printing process with said water-based ink composition. 65.Marking on a document manufactured by screen, flexo or gravure printingwith the ink composition according to claim
 26. 66. Marking on adocument manufactured by screen, flexo or gravure printing with an inkformulation comprising an optically variable pigment according to claim51.
 67. Document carrying a marking according to claim 65.